Those restrictions were decided upon in May 2017, when a college-wide report found that "47% of female seniors participating in gender-exclusive clubs experienced some form of non-consensual sexual contact, compared to about 31% of all female seniors at Harvard." The University decided action must be taken.
A report from the committee stated:
As reflected in survey comments, these organizations directly and negatively influence the undergraduate experience for many students who are not themselves members of these organizations.
The committee will gather final input this fall before making a final recommendation to Harvard President Drew Gilipin Faust. Back in 2015, one Harvard final club, the Fox Club, did offer 9 women provisional memberships. Sadly, however, the club is going back to only accepting men after a failed vote to offer women full memberships.
H/T - Time, The New York Times